Effects of Combinations of Cropping Sequences and Biocovers on Yield of Glyphosate-tolerant Corn, Soybean, and Cotton under No-till

Transcription

1 University of Tennessee, Knoxville Trce: Tennessee Reserch nd Cretive Exchnge Msters Theses Grdute School Effects of Combintions of Cropping Sequences nd Biocovers on Yield of Glyphoste-tolernt Corn, Soyben, nd Cotton under No-till Jennifer Elizbeth Noe University of Tennessee - Knoxville Recommended Cittion Noe, Jennifer Elizbeth, "Effects of Combintions of Cropping Sequences nd Biocovers on Yield of Glyphoste-tolernt Corn, Soyben, nd Cotton under No-till. " Mster's Thesis, University of Tennessee, This Thesis is brought to you for free nd open ccess by the Grdute School t Trce: Tennessee Reserch nd Cretive Exchnge. It hs been ccepted for inclusion in Msters Theses by n uthorized dministrtor of Trce: Tennessee Reserch nd Cretive Exchnge. For more informtion, plese contct trce@utk.edu.

2 To the Grdute Council: I m submitting herewith thesis written by Jennifer Elizbeth Noe entitled "Effects of Combintions of Cropping Sequences nd Biocovers on Yield of Glyphoste-tolernt Corn, Soyben, nd Cotton under No-till." I hve exmined the finl electronic copy of this thesis for form nd content nd recommend tht it be ccepted in prtil fulfillment of the requirements for the degree of Mster of Science, with mjor in Plnt Sciences. We hve red this thesis nd recommend its cceptnce: Arnold M. Sxton, Donld D. Tyler, Dennis R. West (Originl signtures re on file with officil student records.) Fred L. Allen, Mjor Professor Accepted for the Council: Crolyn R. Hodges Vice Provost nd Den of the Grdute School

3 To the Grdute Council: I m submitting herewith thesis written by Jennifer Elizbeth Noe entitled Effects of Combintions of Cropping Sequences nd Biocovers on Yield of Glyphoste-tolernt Corn, Soyben, nd Cotton under No-till. I hve exmined the finl electronic copy of this thesis for form nd content nd recommend tht it be ccepted in prtil fulfillment of the requirements for the degree of Mster of Science, with mjor in Plnt Sciences. Fred L. Allen, Mjor Professor We hve red this thesis nd recommend its cceptnce: Arnold M. Sxton Donld D. Tyler Dennis R. West Acceptnce for the Council: Crolyn R. Hodges, Vice Provost nd Den of the Grdute School (Originl signtures re on file with officil student records.)

4 EFFECTS OF COMBINATIONS OF CROPPING SEQUENCES AND BIOCOVERS ON YIELD OF GLYPHOSATE-TOLERANT CORN, SOYBEAN, AND COTTON UNDER NO-TILL A Thesis Presented for the Mster of Science Degree The University of Tennessee, Knoxville Jennifer Elizbeth Noe December 2007

5 DEDICATION This thesis is dedicted to my prents, Ronld nd Lind Noe, who hve supported nd encourged me throughout my studies, nd my brother nd sister-in-lw, Will Noe nd Ktie Knipe-Noe, nd my sister Ktie Noe, for lwys believing in me nd pushing me to be better. ii

6 ACKNOWLEDGMENTS There re mny people tht I m indebted to for their id nd support in my studies in completing my Mster of Science degree. I would first like to thnk Dr. Fred L. Allen, my mjor professor, for his unwvering support, guidnce, nd encourgement during the preprtion of my thesis. Without him, I would not be here, nd he ws friend nd supervisor t the sme time. I wish to thnk Dr. Arnold M. Sxton, Dr. Don D. Tyler, nd Dr. Dennis R. West for greeing to serve on my thesis committee nd for criticlly reviewing this thesis. Their knowledge nd ptience re immense, nd I m grteful. I would lso like to cknowledge mny people whose ssistnce should not go unnoticed. I would like to thnk Richrd Johnson, Json Wight, nd Rchel Grindle for their help nd friendship over the pst two yers. I would like to thnk ll of the personnel t the Reserch nd Eduction Center t Miln nd the Middle Tennessee Reserch nd Eduction Center t Spring Hill, Tennessee, for their ssistnce in the field work nd the gthering of dt for this experiment. I wnt to express my grtitude to ll of the fculty, stff, nd students of the Deprtment of Plnt Sciences, especilly Debbie Ellis nd Ctherine Nyinyi, for their support nd friendship whenever I needed it. Lst but definitely not lest, I wish to thnk ll of my fmily nd friends for their ptience nd encourgement during the preprtion of this thesis, especilly Ktie Noe, Jne Howell, Neh Krndikr, nd Drew Jeffers. Without them, this experience would not hve been possible. iii

7 ABSTRACT No-till crege is incresing in the United Sttes s producers begin to recognize the environmentl nd economic benefits of this mngement system. Although the potentil to receive crbon credits or pyments for mintining or inititing no-till my encourge producers to employ these prctices, crop yields will be fctor in mngement decisions. Our objective ws to exmine the effects of combintions of cropping sequences nd winter biocovers upon glyphoste-tolernt corn, cotton, nd soyben yields under long-term no-tillge t two loctions in Tennessee. Reserch ws conducted during the first four-yer phse ( ) of two-phse gronomic systems study ( ) t the Reserch nd Eduction Center t Miln (RECM) in Miln, Tennessee, nd the Middle Tennessee Reserch nd Eduction Center (MTREC) in Spring Hill, Tennessee. The experiment ws rndomized complete block design with split block tretments, with the min plots consisting of 13 different cropping sequences of corn, cotton, nd soyben t RECM nd eight different cropping sequences of corn nd soyben t MTREC. The subplots consisted of hiry vetch, whet, poultry litter nd fllow biocovers pplied perpendiculr to the sequences. Rotted corn nd soyben yields were comprble to or higher thn their respective monoculture sequences t both loctions. At RECM, cotton yields took longer to respond to rottion s rotted cotton yields only outperformed continuous cotton in the finl yer. At both sites, corn nd soyben yields were highest under fllow nd poultry litter biocovers, respectively. Cotton yields were highest under poultry litter t RECM. Interction effects of cropping sequence x biocovers were inconsistent s interction effects were only observed on corn yields t MTREC in 2004 nd Hiry vetch, whet, nd poultry incresed yields iv

8 when chnging from continuous corn to rotted corn. The presence of some cropping sequence x biocover interction effects is encourging nd perhps suggests longer time period my be needed for the combined effects of crop sequence nd biocovers upon crop yields to become pprent. Results from the next four yers of the experiment will provide more informtion on the long-term effects of crop sequence nd biocovers on corn, soyben, nd cotton yields. v

9 TABLE OF CONTENTS CHAPTER PAGE Introduction..1 Literture Review: Crop Rottion...3 Literture Review: Biocovers...7 PAPER: Effects of Combintions of Cropping Sequences nd Biocovers on Yield of Glyphoste-tolernt Corn, Soyben, nd Cotton under No-till Abstrct Introduction nd Literture Review Mterils nd Methods..19 Site Description nd Experimentl Design...19 Field Opertions.21 Pesticides nd Fertilizers...23 Sttisticl Anlysis Results nd Discussion...26 Crop Sequence Min Effects...26 Reserch nd Eduction Center t Miln (RECM)...26 Middle Tennessee Reserch nd Eduction Center (MTREC).32 Biocover Min Effects...35 Reserch nd Eduction Center t Miln (RECM)...35 Middle Tennessee Reserch nd Eduction Center (MTREC) 40 Crop Sequence x Biocover Interction Effects 43 Reserch nd Eduction Center t Miln (RECM)..43 vi

10 Middle Tennessee Reserch nd Eduction Center (MTREC) Conclusions...53 References..55 Conclusion..60 Literture Cited..64 APPENDIX A1. Herbicide Mngement Progrm for Corn t the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC)..73 A2. Herbicide Mngement Progrm for soyben t the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC)..73 A3. Herbicide Mngement Progrm for Cotton t the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC)..74 A4. Adjusted nitrogen ppliction rtes for fllow (F), hiry vetch (HV), whet (W), nd poultry litter (PL) biocovers t the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC) from 2002 to A5. Corn (C), soyben (S), nd cotton (T) yields for ll sequence nd biocover combintions t the Reserch nd Eduction Center t Miln (RECM) from 2002 to A6. Corn (C) nd soyben (S) yields for ll sequence nd biocover combintions t the Middle Tennesee Reserch nd Eduction Center (MTREC) from 2002 to Vit...79 vii

11 LIST OF TABLES TABLE PAGE 1. Corn (C), soyben (S), nd cotton (T) rottion sequences t Reserch nd Eduction Center t Miln (RECM) from 2002 to b. Corn (C) nd soyben (S) rottion sequences t the Middle Tennessee Reserch nd Eduction Center (MTREC) from 2002 to A1. Herbicide mngement progrm for corn t RECM nd MTREC...73 A2. Herbicide mngement progrm for soyben t RECM nd MTREC...73 A3. Herbicide mngement progrm for cotton t RECM...74 A4. Adjusted nitrogen ppliction rtes for fllow (F), hiry vetch (HV), whet (W), nd poultry litter (PL) biocovers t the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC) from 2002 to A5. Corn (C), soyben (S), nd cotton (T) yields for sequence/biocover combintions t the Reserch nd Eduction Center t Miln (RECM) from 2002 to A6. Corn (C) nd soyben (S) yields for sequence/biocover combintions t the Middle Tennessee Reserch nd Eduction Center (MTREC) from 2002 to viii

12 LIST OF FIGURES FIGURE PAGE 1. Continuous crop yields verged over ll biocovers t both the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC) for 2002 to Corn (C), soyben (S), nd cotton (T) yields by sequence verged over biocovers t the Reserch nd Eduction Center t Miln (RECM) for 2002 to Corn (C) nd soyben (S) yields by sequence verged over ll biocovers t the Middle Tennessee Reserch nd Eduction Center (MTREC) from 2002 to Corn nd soyben yields by biocover verged over ll sequences t the Reserch nd Eduction Center t Miln (RECM) from 2002 to b. Cotton yields by biocover verged over ll sequences t the Reserch nd Eduction Center t Miln (RECM) from 2002 to Corn nd soyben yields by biocover verged over ll sequences t the Middle Tennessee Reserch nd Eduction Center (MTREC) from 2002 to Interction effects of crop sequence x biocover for those sequences beginning with corn (C) nd soyben (S) t the Reserch nd Eduction Center t Miln (RECM) from 2002 to b. Interction effects of crop sequence x biocover for those sequences beginning with cotton (T) t the Reserch nd Eduction Center t Miln (RECM) from Crop sequence x biocover interction effects for corn (C) nd soyben (S) yields t the Middle Tennessee Reserch nd Eduction Center (MTREC) from 2002 to ix

13 INTRODUCTION At time when globl climte chnge nd rising gs prices re dominnt topics in the United Sttes, the mount of no-till crege is incresing in the United Sttes. This is perhps no coincidence s there is gret interest in no-till in prt due to its potentil to help llevite the greenhouse gs effect by sequestering crbon in the soil (West nd Post, 2002). Also, government progrms tht issue green pyments for conservtion prctices such s no-till s well s the reduced input costs ssocited with this system cst no-till s vible nd profitble mngement system. While the possibility to receive pyments for conservtion prctices is definitely one motivtion for producers to switch to no-till system, mximizing crop yields nd thus profits will lwys be of primry importnce to producer. Mximizing crop yields hs become even more importnt recently due to emphsis upon new biofuels inititives tht will increse demnd for certin crops such s corn, s evidenced by 24% increse in corn production from 2006 to 2007 (NASS, 2007). However, the gol of ny successful mngement system such s no-till is for it to not only be productive in terms of crop yields but tht productivity must be sustinble. Therefore, there is need for reserch on crop yields in long-term no-till systems. Cropping sequences nd biocovers hve been shown to reduce weed nd pest popultions (Fisk et l., 2001; Howrd et l., 1998), reduce soil erosion, improve wter ggregte stbility (Villmil et l., 2006), nd increse soil orgnic crbon levels (Vrvel, 2006; Sinju et l., 2002). Cropping sequences nd biocovers re often importnt prts of no-till systems becuse of these proven benefits. Crop rottions hve been shown to increse yields in crops such s corn nd soyben (Pedersen nd Luer, 2003; Kelley et 1

14 l., 2003). Previous reserch hs lso shown tht the use of biocovers cn lso serve to increse yields of crops such s soyben nd cotton (Adeli et l., 2005; Reddy et l., 2004). However, more reserch is needed on the effects of combintions of cropping sequences nd biocovers on crop yields in long-term no-till systems. As prt of this study, the min objectives were to exmine the effects of cropping sequences, biocovers, nd combintions of cropping sequences nd biocovers on corn, soyben, nd cotton yields t two loctions in Tennessee. Yield dt were gthered nd nlyzed from the first four yers ( ) of long-term no-till study ( ). This study will serve to quntify the effects of cropping sequences nd biocovers upon corn, soyben, nd cotton yields in long-term no-till. 2

15 LITERATURE REVIEW Crop Rottion Crop rottion hs repetedly been shown s n effective mens for incresing crop yields. Explntions for this positive influence of rottions re vried. Since soil conditions cn directly impct crop yield, the influence of crop rottions on soil properties hs been the subject of numerous studies. Incresed soil orgnic mtter levels in no-till production systems hve been observed due to decresed erosion nd to decresed microbil oxidtion (Peterson et l., 1998). Likewise, crop rottion cn lso ffect the nutrient vilbility in soils, especilly in reltion to crbon nd nitrogen. The specific crops in the rottion ffect crbon nd nitrogen levels in lrge prt due to the mount of biomss produced. Biomss represents n input of crbon into the soil. Therefore, those rottions tht include crops tht produce high mounts of residue cn increse soil orgnic mtter nd promote soil productivity (USDA, 2003). For exmple, reserch hs demonstrted tht sorghum/soyben rottion s well s continuously grown sorghum hd higher soil crbon levels thn soyben/soyben rottion due to greter mounts of residue produced by grin sorghum (Hvlin et l., 1990). Other studies hve reveled tht rottions tht include winter cerel crops cn increse soil orgnic mtter when compred to fllow rottions (Cmpbell nd Zentner, 1993). Corn produces more biomss thn soyben nd cotton. Rottions with corn often see incresed crbon levels becuse of this lrge biomss production. In one study, cotton/corn rottion incresed soil orgnic crbon compred to continuous cotton, minly due to the mount of corn biomss produced. In the sme study, however, continuous corn lso showed comprble orgnic crbon levels to the cotton/corn rottion, primrily due to the 3

16 mount of biomss creted from the constnt corn production (Reddy et l., 2006). Other studies show continuous corn under no-till hving higher concentrtions of crbon in the top lyers of the soil in comprison to corn/soyben rottion becuse of the biomss produced yer fter yer (Omonode et l., 2006). Crop rottion cn lso influence the mount of soil orgnic nitrogen. Nitrogen is n importnt nutrient tht, when deficient, cn ffect crop growth nd subsequent yield. Once gin, s seen with crbon, the types of crops nd the mount of biomss produced by those crops in the rottion ffect nitrogen levels in the soil. Orteg et l. (2002) mintined tht greter mounts of crop biomss incresed nitrogen levels in surfce soils. Similr to crbon, soyben rottions with whet nd grin sorghum resulted in greter concentrtions of nitrogen when compred to continuous soyben (Kelley et l., 2003). Including legumes in the rottion cn hve significnt effect on vilble soil nitrogen s they serve to fix nitrogen nd mke it more vilble for the next crop. No-till, with its undisturbed residues, cn increse soil moisture levels when compred to conventionl till (Blevins et l., 1983). The residues decrese wter evportion from the soil surfce s well s decrese wter runoff. When utilizing crop rottions, biomss once gin cn benefit crop growth nd yields by preventing wter evportion nd runoff, therefore incresing the mount of plnt vilble wter. When it comes to crop rottion in no-till system, it is not the mount of soil wter tht is conserved but the efficiency with which tht wter is used for crop growth tht is importnt. More efficient wter use cn llow crop to increse yields without incresing wter use. For exmple, Crookston et l. (1991) demonstrted tht soyben following corn incresed wter use efficiency of soyben. However, corn following 4

17 soyben, while still incresing yields, did so by incresing wter cpcity nd not incresing wter efficiency. In nother study conducted in the Gret Plins, whet wter use efficiency ws greter following grin sorghum thn whet (Schlegel et l., 2002). Reserch hs shown tht rottion cn improve the wter use efficiency of some crops, but this improvement is specific s to rottion order (Anderson, 2005). Crop rottion cn lso serve to counterct the loss of weed, disese, nd insect control ssocited with the loss of tillge. A tremendous mount of selection pressure for surviving weeds exists in monoculture system. Weeds become dpted to the prticulr mngement system nd cn be detrimentl to crop yields. Rotting crops cn serve to lter the environment tht weeds re dpted to nd reduce weed popultions (Higgs et l., 1990). Reductions in pest nd disese dmge hve lso been linked to rotting crops. Severl studies hve shown reduction in soyben cyst nemtode popultions when soyben is rotted with nonhost crop such s corn (Howrd et l., 1998; Chen, et l., 2001). Young et l. (2004) observed tht t lest two consecutive yers of corn should be included in cotton rottion to reduce reniform nemtode numbers. Sclerotini stem rot in soybens nd Fusrium hed blight in whet re both diseses shown to hve reduced incidence when crop rottion is prt of the mngement system (Kurle et l., 2001; Dill- Mcky et l., 2000). Crop rottion serves to decrese disese nd pest popultions in the sme mnner s weed popultions, disrupting disese nd pest life cycles by introducing nonhosts into the rottion. Disese-cusing fungi nd pests cnnot survive in these nonhosts, nd popultion numbers decrese ccordingly. With ll of the benefits ssocited with crop rottion nd incresed soil productivity in mind, it is now useful to exmine the effects of crop rottion on yields. 5

18 Previous reserch hs shown tht crop rottions increse yields of corn, soybens, nd cotton when compred to continuous cropping yields (Crookston et l., 1991; Porter et l., 1997; Reddy et l., 2006). However, this rottion effect is ffected by mny fctors, such s the frequency of certin crops within the rottion nd soil type. The frequency of certin crops in the rottion cn influence crop yields. Reserch hs shown tht nnul corn/soyben rottions frequently show n increse in crop yields compred to their respective monoculture sequences (Porter et l., 1997; Mnnering nd Griffith; Meese et l., 1991). Cotton hs lso responded to rottion, but monoculture cotton is still prominent production prctice tody (Mitchell nd Entry, 1998; Wesley et l., 2001; Reddy, 2006). Rotting with less profitble crop s well s yield uncertinty hs cotton frmers unsure of the benefits of crop rottion. Some studies hve suggested tht crop yields could further be improved by using longer rottions, perhps by dding third crop to the rottion. For instnce, Crookston et l. (1991) sw increses in corn nd soyben yields following five consecutive yers of soyben nd corn, respectively, when compred to n ltenting corn/soyben rottion. Crookston concluded tht dding third crop to the rottion could llow ech crop to hve tht 1 st yer effect, resulting in incresed yields. The previous crop seems to hve n effect upon the yield of the next crop, nd corn nd soyben pper to yield less when following themselves. On the other hnd, study exmining the vibility of corn/soyben/whet rottion found tht there ws no yield dvntge to three-crop rottion, suggesting tht longer time ws needed between sme crop plntings in crop rottion (Lund et l., 1993). Griffith et l. (1988) found tht on high orgnic mtter soils, rotted corn yields under no-till were less thn those in conventionl till. In contrst, rotted corn yields 6

19 under no-till were greter thn conventionl till on low orgnic mtter soil. Soil dringe is nother key fctor ffecting crop yields. Poorly drined soils negtively ffected rotted soyben yields under no-till when compred to rotted soyben under conventionl till (Dick, et l., 1991). However, the negtive impcts of high orgnic mtter nd poor soil dringe on rotted yields under no-till were llevited with time (Griffith et l., 1988; Dick et l., 1991). These results seem to suggest tht long-term notill productions systems combined with crop rottion serve to improve soil conditions so tht yields re equl to or better thn those under conventionl till regrdless of soil type. Biocovers Biocovers re often included in frm production systems to provide soil cover nd to promote soil fertility. By covering the soil, erosion is reduced nd soil productivity is incresed becuse soil orgnic mtter is conserved in the upper soil lyers tht re the most susceptible to erosion. These benefits re enhnced under no-till systems tht leve previous crop residues on the soil surfce. In ddition to preventing crbon loss from erosion, cover crop biomss is incorported into the soil nd serves to increse soil orgnic crbon. A study conducted by Sinju et l. (2002) found tht no-till tomtoes with hiry vetch cover crop incresed soil orgnic crbon levels compred to conventionl till tomtoes without vetch. Similrly, bicultures of rye nd legume cover crops grown with cotton nd grin sorghum incresed soil crbon levels compred to monoculture cover cropping of rye nd grin sorghum, due in prt to higher biomss produced by the biculture cover crops (Sinju et l., 2005). Corn/soyben rottions tht include both vetch nd rye s winter covers increse soil orgnic mtter compred to tht 7

20 of just winter fllow corn/soyben rottion (Villmil et l., 2006). These increses in soil orgnic mtter directly influence other soil properties such s soil ggregtion. Soil ggregte stbility is importnt in terms of wter infiltrtion nd overll soil structure. Increses in soil orgnic mtter cn led to incresed soil ggregtion (Tisdll nd Odes, 1980). Therefore, it is not unusul to see reserch demonstrting tht biocovers, with their bility to increse soil orgnic mtter, hve positive effect upon soil ggregtion. In study compring the effects of winter whet nd dndelion upon soil ggregtion, winter whet incresed soil ggregtion compred to fllow plots (Krbir nd Koide, 2000). Stbiliztion of soil ggregtes by the use of biocovers cn increse the wter content of soils nd thus positively impct crop growth nd yields. Wter ggregte stbility is incresed in corn/soyben rottions tht include vetch nd rye compred to fllow (Villmil et l., 2006). Incresed wter ggregtion is not just restricted to winter cover crops, however. Composted cow mnure pplictions lso improved soil ggregtion when compred to soils tht received no compost in one Cndin study (Whlen et l., 2003). Perhps one of the most importnt contributions of biocovers under ny cropping system is their impct on nitrogen vilbility in the soil. Nitrogen is n essentil nutrient in terms of crop growth nd yield nd plys criticl role in corn, soyben, nd cotton production systems. A legume biocover such s hiry vetch cn serve to increse the nitrogen vilbility for the succeeding crop due to its nitrogen-fixing cpbilities, fulfilling prt of the nitrogen requirements of the crop (Schwenke et l., 2001). Poultry litter, biocover consisting of poultry mnure nd bedding mterils s source of nitrogen nd other nutrients, cn lso impct the mount of nitrogen vilble for plnt 8

21 growth. Studies compring nitrogen ccumultion using poultry litter nd mmonium nitrte hve demonstrted higher levels of totl nitrogen s well s nitrte in the surfce lyers of the soil with poultry litter use (Nyktw et l., 2001b; Mitchell nd Tu, 2006). There re concerns s to nitrte ccumultion when poultry litter is used, but some studies hve shown tht the use of rye cover crop cn prevent the buildup nd subsequent leching of nitrte in nd from soils (Nyktw et l., 2001; Nyktw et l., 2001b). As result, cerel cover crops hve the potentil to not only prevent nutrient loss due to nitrte leching but lso to prevent the environmentl problems ssocited with nitrte contmintion of groundwter. Cerel cover crops such s rye nd winter whet cn reduce nitrte leching by using the nitrogen from the preceding crop to supplement their own growth (Strock et l., 2004; Cost et l., 2000). A combintion of rye nd vetch winter covers in no-till corn/soyben rottion yielded less soil nitrte thn found in the sme corn/soyben system without cover crops (Villmil et l., 2006). However, cerel cover crops do not efficiently return this nitrogen to the soil, nd the next crop might not benefit from this retined nitrogen. Biocovers lso hve dvntges in terms of improved weed nd insect control. Severl studies hve documented reduced weed emergence when using cover crops such s rye, hiry vetch, nd clover in corn nd soyben production systems (Fisk et l., 2001; Reddy et l., 2003; Reddy et l., 2004). Brssic cover crops such s cnol cn reduce weed emergence in comprison to fllow tretments (Hrmoto, 2005). Biocovers compete with weeds for resources such s light, wter, nd nutrients nd prevent their estblishment. Insect popultions cn lso be reduced with the use of biocovers, primrily becuse biocovers provide hbitts for beneficil insects (Cremer 1999; 9

22 Tillmn et l., 2004). These beneficil insects seek out insect pests nd regulte pest popultion levels. Since biocovers cn ffect soil crbon nd nitrogen levels, soil structure, nd pest popultion levels, they cn lso ffect crop yields tht re directly impcted by chnges in these fctors. Reserch shows vried results when exmining the effects of biocovers upon crop yields, with the differences vrying ccording to the cover crop chosen. Corn yields hve been reduced fter rye cover crop but incresed fter hiry vetch (Vughn nd Evnylo, 1998; Johnson et l., 1998). Lower cotton yields hve been observed when nother cerel cover crop, winter whet, ws used but incresed with hiry vetch s cover crop (Lrson et l., 2001). These results indicte the importnce of utilizing biocovers tht provide much-needed nitrogen to corn nd cotton. Since nitrogen is criticl to soyben production, one would expect to see the sme decresed yields lso seen in corn nd cotton when rye or winter whet cover crop is used. In contrst, soyben yields fter rye were observed to increse when compred to soyben in fllow rottion. De Bruin et l. (2005) found tht soyben yields in rye rottion were equl to those in fllow but not s profitble. Soyben yields re generlly not ffected by cerel crops unless wter is limiting fctor. Poultry litter pplictions hve lso been shown to benefit crop yields by supplying nitrogen s well. Cotton yields hve incresed when winter rye cover crop with poultry litter s nitrogen source ws used compred to cotton/fllow yields (Reddy et l., 2004). Soyben yields hve lso been observed to benefit from poultry litter ppliction (Adeli et l., 2005). Numerous studies seem to focus on the nutrients tht biocovers cn supply to succeeding crops, 10

23 indicting tht those biocovers tht supply the most nutrients such s nitrogen will result in incresed yields. 11

24 EFFECTS OF COMBINATIONS OF CROPPING SEQUENCES AND BIOCOVERS UPON YIELD OF GLYPHOSATE-TOLERANT CORN, SOYBEAN, AND COTTON UNDER NO-TILL 12

25 Abstrct Crop sequencing nd biocovers re often integrl prts of no-till systems becuse of benefits such s improved pest control nd incresed soil orgnic mtter. The effects of cropping sequences nd biocovers on corn, soyben, nd cotton yields were studied t two loctions in Tennessee. The experimentl design ws rndomized complete block with split block tretments. The min plots were 13 different cropping sequences of corn, cotton, nd soyben t the Reserch nd Eduction Center t Miln (RECM) nd eight different cropping sequences of corn nd soyben t the Middle Tennessee Reserch nd Eduction Center (MTREC). The subplots consisted of hiry vetch, whet, poultry litter, nd fllow biocovers pplied perpendiculrly to the cropping sequences. Corn hd slight response to rottion t both loctions, but the response of soyben ws much stronger s rotted soyben yields were generlly higher thn continuous soyben yields. Cotton did not respond well to rottion s continuous cotton yields verged higher thn rotted cotton yields in the first three yers. Four-yer verges of corn yields were high under fllow biocover, producing 9.79 nd 6.21 Mg h -1 t RECM nd MTREC. Fouryer verges of soyben (RECM, 3.57 Mg h -1 ; MTREC, 2.85 Mg h -1 ) nd cotton yields (RECM, 1.01 Mg h -1 ) were highest under poultry litter. Cropping sequence x biocover interction effects were only observed on corn yields t MTREC. Hiry vetch nd whet biocovers combined with rottion were the most consistent in incresing corn yields. Results from the next experimentl phse my better reflect the effects of cropping sequences nd biocovers upon corn, soyben, nd cotton yields. 13

26 CHAPTER 1 Introduction nd Literture Review Conservtion tillge, or those tillge methods in which 30% or more of the soil surfce is covered by plnt residue, is incresingly being dopted by producers (SSSA, 2007). No-till, conservtion tillge system in which crop is plnted directly into the previous crop s residue without prior tillge, is used in this study becuse of the growing number of no-till crege throughout the United Sttes. This is trend tht is lso reflected in the incresing mount of no-till crege found in Tennessee. From 2000 to 2004, totl no-till crege rose from 55.6% to 58.4% of the totl crege plnted in Tennessee (NASS, 2004). In 2004 lone, the proportion of soyben [Glycine mx (L.) Merr.], corn [Ze mys L.], nd cotton [Gossypium hirsutum L.] crege plnted in Tennessee ws 67.8, 64.3, nd 47.4%, respectively (NASS, 2004). Efforts to persude more producers to switch to this type of production system should consist of not just conveying the obvious informtion in terms of reduced input costs but lso conveying the other potentil economic nd environmentl benefits ssocited with the use of no-till. Due to concerns bout the effects of globl climte chnge, incresing emphsis is being plced upon finding methods with which to reduce the emission of greenhouse gses such s crbon dioxide into the tmosphere. The disruption of the soil with tillge promotes the relese of crbon dioxide into the tmosphere. Thus, no-till hs been proposed s wy to sequester crbon into the soil nd prevent its relese (Ll, 1997; West nd Post, 2002). The Kyoto Protocol hs estblished process for globl 14

27 crbon credits system, but there re doubts s to its effectiveness (Kirschbum et l., 2001; Mrlnd et l., 2001). However, the potentil to receive crbon credit pyments is one incentive for producers to switch to no-till s producers cn ern up to two dollrs for every cre under no-till (Ntionl Frmers Union, 2007). In ddition to receiving crbon credits, switching to no-till might lso llow the producer to be eligible to receive government pyments for production prctices tht benefit the environment. No-till hs been proven to improve soil conditions nd soil fertility, in prt becuse residues, left on the soil surfce, hve n impct on soil qulity nd thus plnt growth. Reduced soil erosion, incresed ggregte stbility, nd higher soil orgnic mtter (SOM) hve ll been ssocited with no-till when compred to conventionl till systems (Schuller et l., 2007; Kldivko, 1986; Rhoton, 2000; Thoms et l., 2007). The residues left from the previous crop serve to protect the soil from rin dmge, to retin soil moisture, nd to prevent crbon from escping from the soil. As result, improved soil conditions mke more suitble environment for crop growth. Reserch hs shown mixed results when compring no-till crop yields to conventionl crop yields, with the results vrying ccording to the crop. Specificlly, soyben yields from no-till hve been found to be equl to or better thn those under conventionl tillge (Tyler et l., 1983; Hussin et l., 1999). Vyn et l. (2000) observed tht soyben yields under no-till were initilly lower thn those under conventionl till; however, over 25-yer period, yield reductions were not significnt. Findings similr to this 25-yer study could be importnt when exmining the long-term vibility of no-till system. 15

28 For corn nd cotton, yield results re more wide-rnging. Hussin et l. (1999) found tht when grown in rottion with soyben, no-till corn yields were equl to conventionl yields over n eight yer period. Yield nd economic returns were higher for no-till corn thn conventionl returns in semirid environments in Texs nd Mexico (Smrt, 1999). Other studies hve produced similr incresed yield results. On the other hnd, reductions in corn yield hve been seen in irrigted continuous corn under no-till with low nitrogen fertiliztion (Sims et l., 1998). No-till cotton yields reflect the sme inconsistency in returns s no-till corn. Cotton hs produced similr or greter yields to those chieved under conventionl tillge, but lower yields due to delyed development hve lso been observed (Schwb et l., 2002; Triplett, 1996; Pettigrew nd Jones, 2001). Aside from vrying crop yields, no-till does hve other drwbcks when compred to conventionl till s well. The use of no-till elimintes the benefits tht come with tillge, primrily in the forms of reduced weed, disese, nd pest control. As result, herbicides re more hevily relied upon, nd ny incresed returns from eliminting tillge costs my be countercted by higher herbicide costs (Smith et l., 1992). Crop rottion nd biocovers become importnt elements in no-till system to compenste for these problems. Crop rottion cn disturb the growing environment of weeds so tht weeds re no longer ble to thrive in the sme mnner s under continuous cropping. Biocovers serve to further protect the soil from erosion, increse soil orgnic mtter nd soil stbility, provide some weed control, improve crop nutrition, nd increse crop yields (Snpp, 2005). Crop rottion hs been shown to be n effective wy to increse crop yields. In one three yer-study, Pedersen nd Luer (2002) found tht corn-soyben rottion 16

29 resulted in 12% higher corn yields thn continuous corn. Griffith et l. (1988) observed 20% greter rotted corn yields under no-till compred to no-till continuous corn yields. Other crops hve lso benefited when in rottion s compred to monocropping. Conventionlly tilled soybens in rottion with grin sorghum yielded higher thn continuous soyben in reserch conducted by Wesley et l. (2001) in Mississippi. Wilhelm nd Wortmnn (2004) found tht soyben in n lternting soyben-corn rottion yielded higher thn continuous soyben in 16-yer study. Conventionlly tilled rotted cotton yields were lso higher thn yields of continuous cotton in the sme study. Previous reserch hs lso shown tht the use of biocovers cn enhnce crop yields s well. The bility of biocovers to supply nutrients to succeeding crops is fctor in incresed crop yields. No-till corn yields were higher fter legume cover crops such s hiry vetch when compred to fllow corn yields in two studies by Blevins et l. (1990) nd Decker et l. (1994). Corn yields most likely benefited from the extr nitrogen provided by the legume biocovers. Soyben yields with rye cover crop were equl to those with no cover crop (Reddy et l., 2003), nd poultry litter pplictions hve been shown to benefit soyben yields s well (Adeli et l., 2005). For no-tillge cotton with no nitrogen fertiliztion, yields were incresed with hiry vetch crop but decresed with winter whet cover crop (Lrson et l., 2001). Conversely, Prvin et l. (2004) sw incresed cotton yields with whet cover crop compred to no cover crop in study performed under no-till conditions. Given tht crop rottions nd winter covers cn llevite some of the problems ssocited with no-till s well s improve crop yields, reserch into their combined effects on crop yields in no-till system is necessry to mke good mngement 17

30 recommendtions tht will improve corn, soyben, nd cotton yields nd benefit the producers in terms of improved soil qulity nd higher profits. While there re studies tht primrily focus on just no-till systems, the ones tht look t the effects of cropping sequences nd biocovers on yields only incorporte smll number of sequences nd biocovers nd re reltively short-term experiments. Therefore, the primry objectives of this study re to determine: i) the effects of vrious cropping sequences, ii) the effects of biocovers, nd iii) the effects of combintions of cropping sequences nd biocovers upon corn, soyben, nd cotton yields under long-term no-till mngement system. 18

31 CHAPTER 2 Mterils nd Methods Site Description nd Experimentl Design Field reserch ws conducted during the first four-yer phse ( ) of twophse systems study ( ) t both the Reserch nd Eduction Center t Miln (RECM) in Miln, Tennessee, nd the Middle Tennessee Reserch nd Eduction Center (MTREC) ner Spring Hill, Tennessee. Ech loction ws under long-term no-till production system where the crops were plnted directly into the previous crop residue. The Miln study ws conducted on Grend silt lom (fine-silty, mixed, thermic Oxyquic Frglossudlf), nd the Middle Tennessee study ws conducted on Mury silt lom soil (fine, mixed, semictive, mesic Typic Pleudlf). The experimentl design ws rndomized complete block in split block rrngement with four replictions. The min plots consisted of 13 different cropping sequences t RECM nd eight different cropping sequences t MTREC (Tbles 1 nd 1b). The sequences consisted of glyphoste-tolernt corn, soyben, nd cotton t RECM nd glyphoste-tolernt corn nd soyben t MTREC. At RECM, plots were fllow in 2001 prior to plnting. At MTREC, those plots initilly plnted to corn were plnted to soyben in 2001, nd those plots initilly plnted to soyben were plnted to corn in The subplots t both loctions consisted of four different biocovers being pplied perpendiculr to the cropping sequences. The four biocovers were fllow, hiry vetch, 19

32 Tble 1. Corn (C), soyben (S), nd cotton (T) rottion sequences t the Reserch nd Eduction Center t Miln (RECM) from 2002 to Yer Crop sequence T S T C 2 T T T T 3 T S C T 4 T C T S 5 T C T C 6 C C C C 7 C T S 8 C C S 9 C S C S 10 S S S S 11 S S C T 12 S T C S 13 S T S T C T Tble 1b. Corn (C) nd soyben (S) rottion sequences t the Middle Tennessee Reserch nd Eduction Center (MTREC) from 2002 to Yer Crop sequence C S C S 2 C C S C 3 C C C C 4 C S S C 5 S S S S 6 S C S C 7 S C C S 8 S S C S 20

33 poultry litter, nd winter whet. A totl of 208 plots were studied t RECM, nd totl of 128 plots were studied t RECM. Field Opertions In 2002, four different corn hybrids nd four different soyben vrieties were plnted t both MTREC nd RECM. The four corn hybrids used were Terrl TV 2140RR, Biogene BG 098RR, Deklb DKC 6410RR, nd Deklb DKC 60-09RR. The four soyben vrieties used were USG 7440nRR, Delt King DK 4461RR, Asgrow AG 3702RR, nd Asgrow AG 3701RR. At RECM, the five different cotton vrieties plnted were Sure-Grow 215BG/RR, Pymster 1218 BG/RR, Delt & Pine 436RR, Stoneville 4793RR, nd Stoneville 4892BG/RR. Due to discontinution of the lines, only one corn hybrid (Deklb DKC 6410RR ), one soyben vriety (USG 7440nRR ), nd one cotton vriety (Pymster 1218BG/RR ) ws plnted in For ll four yers, the continuous corn, soyben, nd cotton sequences were plnted with Deklb DKC 6410RR, USG 7440nRR, nd Pymster 1218 BG/RR, respectively. Biocovers were plnted in or round October of the previous yer nd burned down prior to plnting. Corn, soyben, nd cotton plots were plnted t RECM using 4-row John Deere 7600 Mxemerge plnter nd t MTREC using 6-row John Deere plteless plnter (Deere & Compny, Moline, IL). At both loctions, corn, soyben, nd cotton plots were plnted t recommended University of Tennessee seeding rtes of 64,247 seeds h -1, 258, ,445 seeds h -1, nd 64,495 seeds h -1, respectively. Corn nd soyben plots were hrvested t RECM with n AC Glener combine (AGCO, Duluth, GA) in 2002 nd with two-row ALMACO (ALMACO, Nevd, IA) combine in 2003, 21

34 2004, nd 2005 nd t MTREC using K-2 AC Glener combine with three-row heder nd 3.05 m grin pltform. At RECM, corn plots were plnted with cm row spcing in 6.10 m x m plots, creting 8-row plots. Corn plots were plnted on 12 April 2002, 28 April 2003, 29 April 2004, nd 9 My Corn plots were hrvested on 29 August 2002, 19 September 2003, 24 September 2004, nd 14 September Two rows were hrvested in ech yer. At MTREC, corn plots were plnted with cm row spcing in 6.10 m x m plots, creting 8-row plots. Corn plots were plnted on 16 April 2002, 28 April 2003, 23 April 2004, nd 5 My In 2005, corn plots were replnted on 18 My becuse of poor emergence due to combintion of dry wether nd niml dmge. Corn plots were hrvested on 9 September 2002, 15 September 2003, 27 September 2004, nd 23 September Two rows were hrvested in 2002, 2003, nd 2004 nd ten rows were hrvested in The mesurements tken t both loctions t the time of hrvesting were plot weights in pounds nd percentge moisture content. Corn plot weights were djusted to Mg h -1 nd stndrd moisture content of 155 g kg -1 for further dt nlysis. Soyben plots t RECM were plnted with cm row spcing in 6.10 m x m plots, creting 8-row plots. Plnting dtes t RECM were 21 My 2002, 30 My 2003, 25 My 2004, nd 10 My Soyben plots were hrvested on 23 September nd 16 October 2002, 2 October 2003, 24 September 2004, nd 30 September Two rows were hrvested in 2002 nd 2003 nd three rows were hrvested in 2004 nd Soyben plots t MTREC were lso plnted with cm row spcing in 6.10 m x m plots, creting 8-row plots. At MTREC, soyben plots were plnted on 29 April 2002, 22

35 13 My 2003, 17 My 2004, nd 16 My Soyben plots were hrvested on 16 October 2002, 30 September 2003, 3 October 2004, nd 9 September Four rows were hrvested ech yer. The mesurements tken t both loctions t the time of hrvesting were plot weights in pounds nd percentge moisture content. Soyben plot weights were djusted to Mg h -1 nd stndrd moisture content of 130 g kg -1 for further dt nlysis. Cotton plots were plnted only t RECM. Cotton plots were plnted with cm row spcing in 6.10 m x m plots, creting 6-row plots. Cotton plots were plnted on 7 My 2002, 12 My 2003, 5 My 2004, nd 11 My Cotton plots were hrvested on 10 September 2002, 16 October 2003, 8 November 2004, nd 25 October Two rows were hrvested ech yer. Mesurements tken t RECM t the time of hrvesting included plot weights in pounds, lint weights in pounds, nd lint percentges. Plot weights were djusted to Mg h -1 using their respective lint percentges to obtin n djusted plot weight for further dt nlysis. Pesticides nd Fertilizers Before plnting, burn down herbicides were used for removl of existing vegettion nd biocovers t both loctions. Either prqut (1,1-Dimethyl-4,4-bipyridinium), glyphoste (N-(phosphonomethyl)-glycine), or glufosinte mmonium (mmonium(±)- 2mino-4-(hydroxymethylphosphinyl) butnote) were pplied for burn down in April of ech yer before corn, soyben nd cotton seeding. One or two pplictions of glyphoste were pplied to the soyben nd corn plots t both loctions in or round My or June of ech yer. For the cotton plots locted only t RECM, pesticide use ws 23

36 extensive nd ppliction dtes rnged from June through September of ech yer. Glyphoste nd clethodim (RS)-2-9[(E)-1-[(E)-3-chlorollyloxyimino] propyl]-5-[2- (ethylthio) propyli]-3-hydroxycyclohex-2-en-1-l-one) were the most common herbicides used ll four yers. Def (S,S,S-Tributyl phosphorotrithiote), Bidrin (Dimethyl phosphte of 3-Hydroxy-N,N-dimethyl-cis-crotonmide) nd Pix (1,1- dimethylpiperidinium chloride) were lso used for dditionl pest control nd plnt growth regultion. A more complete list of pesticides nd ppliction rtes t both loctions cn be found in Tbles A1, A2, nd A3 in the Appendix. Fertilizer pplictions were mde ccording to University of Tennessee recommendtions. At RECM, brodcst pplictions of mmonium nitrte t rtes rnging from 45 to 90 kg h -1 of nitrogen were pplied specific to cover crops on vrious dtes in Mrch, April, nd My of ech yer. Adjusted nitrogen ppliction rtes for ech biocover re shown in Tble A4 of the Appendix. Additionl sidedressing pplictions of mmonium nitrte t rtes of 112 to 157 kg h -1 of nitrogen were mde to the corn plots in My or June of ech yer. In June of 2004, cotton plots were sidedressed with 34 kg h -1 of nitrogen. Poultry litter ws pplied t rtes rnging from 2241 to 4482 kg h -1 in Mrch or April prior to plnting to chieve 67 kg h -1 of nitrogen bsed on 50% biovilbility. At MTREC, mmonium nitrte ws pplied specific to cover crops ll four yers t rtes of 50 to 67 kg h -1 of nitrogen. Adjusted nitrogen ppliction rtes for ech biocover t MTREC re locted in Tble A4 in the Appendix. Corn plots were sidedressed with mmonium nitrte in My or June of 2002, 2003, nd 2005 t rtes of 123 to 135 kg h -1 of nitrogen. Corn ws topdressed with mmonium nitrte in 2004 t rte of 112 kg cre -1 of nitrogen. Potsh ws pplied to ll plots in April of ech yer t 24

37 rtes rnging from 112 to 168 kg h -1. As t RECM, poultry litter ws pplied t rtes rnging from 2241 to 4482 kg h -1 in Mrch or April prior to plnting to chieve 67 kg h -1 of nitrogen bsed on 50% biovilbility. Sttisticl Anlysis Anlysis of vrince (ANOVA) of the dt ws performed using the MIXED procedure of SAS (SAS, 2003). All yers nd crops were nlyzed seprtely t both loctions. Crop sequence nd biocovers were treted s fixed effects nd repliction s rndom effect. Log trnsformtions of the dt were performed when necessry to stisfy normlity nd homogeneity of vrinces. Dt were then bck-trnsformed, nd bck trnsformed mens nd stndrd errors were reported. Mens were seprted using Fisher s protected LSD (P 0.05), nd SAS mcro PDMIX800 (Sxton, 1998) ws used to convert the men seprtion output to letter groupings. 25

38 CHAPTER 3 Results nd Discussion Crop Sequence Min Effects Reserch nd Eduction Center t Miln (RECM) At RECM, continuous corn, soyben, nd cotton yields were comprble to or higher thn rotted corn, soyben, nd cotton yields s expected in the first yer of the experiment. Both corn nd soyben showed some positive response to rottion, with the response of soyben yields being stronger thn the response of corn yields. Cotton yields responded to rottion only in the fourth yer. The lternting corn-soyben sequence, when comprisons could be mde, hd significntly higher yields thn both continuous corn nd soyben. It ppered tht corn nd soyben responded much quicker to rottion thn cotton, but some higher rotted cotton yields in the fourth yer my indicte tht cotton will respond fvorbly to rottion s the experiment progresses nother four yers. Corn For the most prt, continuous corn yields, when verged over ll biocovers, were higher in 2004 nd 2005 thn in 2002 nd In the fourth yer, continuous corn yields slightly decresed (Fig. 1). However, both soyben nd cotton yields lso decresed in this yer, suggesting tht this ws lower-yielding yer. Sttewide crop yields lso reflected this decrese from 2004 to 2005, s corn yields decresed from 8.81 to 8.72 Mg h -1, soyben yields decresed from 2.75 to 2.55 Mg h -1, nd cotton yields decresed from 1.01 to 0.95 Mg h -1 (NASS, 2007). Continuous corn yields were the sme s or significntly lower thn corn yields in rottion in the lst three yers (Fig. 2). These results were not surprising s previous studies hve shown tht continuous corn 26

39 10 8 Continuous corn Continuous soyben Continuous cotton RECM MTREC Yield, Mg h Yer Fig. 1. Continuous crop yields verged over ll biocovers t both the Reserch nd Eduction Centers t Miln (RECM) nd Spring Hill (MTREC) for 2002 to

40 Corn Soybens Cotton b b Yield, Mg h b b c C-CCC C-SCS C-TSC C-CST S-SSS S-TST S-TCS S-SCT T-TTT T-CTC T-CTS T-SCT T-STC b c c c 2004 b b b CC-CC CS-CS CT-SC CC-ST SS-SS ST-ST ST-CS SS-CT TT-TT TC-TC TC-TS TS-CT TS-TC b 2 c b bc c 0 CCC-C CSC-S CTS-C CCS-T SSS-S STS-T STC-S SSC-T TTT-T TCT-C TCT-S TSC-T TST-C Sequence CCCC CSCS CTSC CCST SSSS STST STCS SSCT TTTT TCTC TCTS TSCT TSTC Fig. 2. Corn (C), soyben (S), nd cotton (T) yields by sequence verged over biocovers t the Reserch nd Eduction Center t Miln (RECM) for 2002 to Men seprtion (Fisher s LSD, P 0.05) only vlid for the sme crop within ech yer. 28

41 yields re often lower thn rotted yields in no-till systems (Lund et l., 1993; Vyn et l., 2000). Significnt corn crop sequence effects (P 0.01) were observed in 2002 nd While significnt corn crop sequence effect ws unexpected in the first yer of the experiment, one explntion could be tht four different corn vrieties were used in this yer, perhps contributing to the significnt sequence effect. In 2004, the lternting corn-soyben rottion yielded significntly higher thn continuous corn, producing Mg h -1 to continuous yields of Mg h -1 (Fig. 2). These results support reserch by Luer et l. (1997) tht found corn-soyben rottion yielded 13% greter thn continuous corn. Rottions in which corn immeditely followed one yer of soyben (CS-C nd TS-C) yielded significntly higher thn corn following two yers of soyben (SS-C) nd corn immeditely following cotton (ST-C) in 2004 s well (Fig. 2). In both yers where comprison could be mde, no differences were found in corn yields of the lternting cotton-corn rottion versus continuous corn. Bruns et l. (2007) found tht corn yields incresed when in rottion with cotton s compred to yields of continuous corn. Even though our results show corn yields in the cotton-corn rottion to be comprble to nd not higher thn the lternting rottion, it my be too erly in the cropping sequence to see the negtive effects of monoculture corn s compred to putting cotton nd soyben in the rottion with corn. Soyben Continuous soyben yields, when verged over ll biocovers, fluctuted from yer to yer. Yields were higher in 2003 nd 2004 compred to 2002 nd 2005 (Fig. 1). In , continuous soyben yields were found to be the sme s or significntly lower thn rotted yields (Fig. 2). These results support previous findings tht soyben yields in rottion re higher thn those yields of continuous soyben (Vyn et l., 2000; 29